The present invention relates to improvements in control of the movement of the document platen in an electrophotographic copying machine of the type in which the document platen is movable with respect to the machine body.
There is known an electrophotographic copying machine having a document platen movable with respect to the machine body. In the electrophotographic copying machine of this type, the document platen and the machine body are generally connected to each other by a slide rail mechanism. As the most commonly used slide rail mechanism, the mechanism under trademark ACCURIDE is known to those skilled in the art.
In the electrophotographic copying machine of this type, there are available two methods of controlling the movement of the document platen. According to one method, the moving distance of the document platen varies with the size of a document, whereas according to the other method, the document platen is moved to a maximum extent at all times regardless of the document size. For making a plurality of reproductions from a document, particularly from a small-size document, the former method is superior in shortened reproduction time to the latter. Accordingly, it is a general practice to employ the former method.
However, the electrophotographic copying machine employing the former method has sometime presented the problem that the slide rail mechanism such as ACCURIDE or the like is not smoothly moved when a larger-size document is reproduced after several reproductions of a smaller-size document have been made.
More specifically, the slide rail mechanism such as ACCURIDE has a longitudinal outer member extending in the moving direction of the document platen, an inner member slidable in the longitudinal direction of the outer member, and an intermediate member inserted between the outer and inner members. The intermediate member has spherical bodies for assuring smooth slide of the inner member with respect to the outer member. Generally, the outer member is attached to the document platen, while the inner member is attached to the machine body. When the document platen is moved, the outer member and the inner member are relatively moved through the intermediate member. However, when a plurality of reproductions are made from a document, for instance from a small-size document, the document platen is repeatedly subjected to advance at a relative slow exposure speed and retreat at a relative fast return speed in a predetermined range. In this case, the intermediate member will fail to follow the movement of the inner and outer members at the time when the movement of the document platen is switched from advance to retreat. This prevents the spherical bodies of the intermediate member from rotating between the outer and inner members, but causes these spherical bodies to slip therebetween. This results in alteration in the positional relationship between the outer member and the intermediate member or between the inner member and the intermediate member. That is, the intermediate member presents migration.
With the intermediate member presenting such migration, when intending to reproduce a document having a size larger than that of the document previously reproduced, the intermediate member presenting the migration is returned to the original state where the intermediate member presents no migration, with the movement of the outer member when the document platen is moved in the exposure direction exceeding the previous small-size document area. This generates sliding friction between the outer member and the spherical bodies of the intermediate member or between the inner member and the spherical bodies of the intermediate member. This applies a load to the document platen under exposure, causing exposure deviation, i.e., deviation of a reproduced image from the image to be reproduced. Thus, the reproduced image is impossible to read, or illegible.